1. Field of the Invention
The present invention relates to a micromachined sensor device for determination of parameters affecting said device. In particular, the present invention relates to a method for monitoring strain variations of a strain responsive element of a micromachined sensor device.
2. Description of Related Art
A method for using a micromachined device is taught in "A pressure responsive device having a resonantly vibrating element in an elastic tube", GB 2 223 582. The method monitors strain variations of a strain responsive element of a micromachined sensor device and comprises the steps of:
(a) irradiating said strain responsive element by a beam of light with at least a frequency swept modulated intensity for activating said element in order to oscillate in a corresponding oscillation resonance mode at its resonance frequency; PA1 (b) determining frequency characteristics of said device when subjected to predefined parameter conditions; PA1 (c) subjecting said element to outside parameter conditions; and PA1 (d) detecting the beam of light after being modified correspondingly by said activated element. PA1 (a) irradiating said element by a beam of light with at least frequency swept modulated intensity for activating said element such that at least two oscillation resonate modes are activated subsequently at their resonance frequencies; PA1 (b) determining frequency characteristics of said device when subjected to predefined parameter conditions by measuring the corresponding resonate frequencies of two said resonate modes at said predefined parameter conditions; PA1 (c) determining correspondingly two parameter values for said device by fitting the measured resonate frequencies upon the corresponding frequency characteristics; PA1 (d) subjecting said element to outside parameter conditions; and PA1 (e) detecting the beam of light after being modified correspondingly by said activated element. PA1 a micromachined sensor device having a strain responsive element, said strain responsive element responding to strain variations resulting from the varying outside parameter conditions; PA1 a light beam supply means functional for irradiating said element with light; PA1 a frequency oscillator means functional for generating a beam of light with frequency swept modulated intensity in order to activate said element in at least two oscillation resonance modes at their resonance frequencies, whereby the oscillator means supplies the beam supply means with an intensity modulated light beam; PA1 detecting means functional for detecting a modified beam of light, said modified beam being light modified by said activated element; PA1 measuring means functional for measuring the resonance frequencies from said modified beam; and PA1 processing means functional for processing resonance frequency values and deriving therefrom at least correspondingly two outside parameter values. PA1 These and other objects and advantages of the present invention will no doubt become apparent to those of skill in the art after having read the following detailed description of the preferred embodiments which are contained herein and illustrated by the various figures.
The method of GB 2 223 582 clearly addresses a micromachined pressure responsive device. For example, it is to be used as a "down well" pressure sensor in oil drilling and exploration. Only pressure response is known for this device. In particular, this method teaches that said micromachined sensors are made from single crystal silicon.
In an article by Andres et al., "Sensitivity and mode spectrum of a frequency-output silicon pressure sensor" Sensors and Actuators 15 (1988) pages 417-426, different vibrational modes of a so-called micromachined, butterfly silicon sensor integral with a thin diaphragm are shown. The modes are investigated by activating electrically and interrogating optically. In particular, for said modes, relationships between resonance frequency and pressure are determined. The modes M.sub.nm are classified in accordance with position and direction of node-axes.
In an article by Uttamchandani et al., "Optically excited resonant beam pressure sensor", Electronic Letters, 3rd December 1987, Vol. 23, No. 25, pages 1333-1334, resonance frequency/pressure and resonance frequency/temperature relationships for the fundamental mode of vibration of a beam-type micromachined silicon resonator are determined. No further interrelationships or dependencies are shown.
Moreover, from the above documents, the usual way of driving such resonators and detecting resonance frequencies by optical activation/interrogation techniques is known. In particular, two light sources, i.e., a pulsed mode source for activation and a continuous-wave source for interrogation are employed. Light transmitted by said sources is joined and guided in one fiber to the sensor, thus avoiding complex instrumental arrangements at the measuring position.
Whereas from the above advantageous use of one single fiber for activation and interrogation of the vibrational modes of such sensors, only a determination of the corresponding one parameter responsible for operation conditions, as mentioned, is realized.
However, in most cases, it is necessary to determine a set of parameters to qualify accurately working conditions in hostile environmental situations. For example, in the above mentioned downhole circumstances, exploration and production activities should be carried out at operation conditions as safe as possible. Especially pressure/temperature combinations should be monitored closely and reliably. Moreover, with respect to production activities, permanently installed sensor devices have to be used for long time periods.
To remedy the shortcomings, as addressed above, several solutions are proposed For example in the article by Vincent et al "An All-optical Single-fibre Micromachined Silicon Resonant Sensor: Towards a Commercial Device", Sensors and Actuators A, 25-27 (1991), pages 209-212, both the use of a set of sensors, called multiplexing sensors, with different resonance frequencies, and mechanical temperature compensation are disclosed.
A further compensation method is shown in "Sensors using Vibrating Elements" EP 371 592. Also, one fiber communication linkage is applied in this reference. The sensor device shown comprises a couple of strain responsive silicon beams, with one being affected by both pressure and temperature variations, the other having a free end only being subjected to temperature variations. Thus temperature corrections can be made.
However, the solutions and options as presented in the documents discussed, either seem to represent only a device development stage, or employ multicomponent sensor devices and related complex or incomplete parameter monitoring programs.